1. Field of the Invention
The present invention relates to an analog audio filter for correcting high frequencies. The present invention more particularly applies to the implementation of an active filter to be integrated in an analog mixer, for example, of a sound card for a microcomputer.
2. Discussion of the Related Art
An active filter for correcting high frequencies must respect a given attenuation contour. FIG. 1 shows, in the form of a gain/frequency characteristic, the attenuation contour of an analog audio filter for correcting high frequencies to which the present invention applies.
The transfer function of the filter has a fixed pole, generally at 3 kHz and 0 dB, and a pole which is a function of the gain of the amplifier. The frequency variation of the variable pole follows a slope of xc2x16 dB per octave from the fixed pole. In the example shown in FIG. 1, the linear gain is 0 dB for frequencies lower than 3 kHz. For a gain of xc2x12 dB, the cut-off frequency is 3.8 kHz. For a gain of xc2x114 dB, the cut-off frequency becomes 15 kHz.
With respect to the application of active filters to sound cards for microcomputers, the filter attenuation contour generally is symmetrical with respect to 0 dB. To implement such an active filter, for amplification or for attenuation, a programmable filter is generally used.
FIG. 2 shows a first example of a conventional assembly for implementing an active filter having an attenuation contour such as shown in FIG. 1.
This assembly uses a differential amplifier 1 mounted as a follower. An analog input signal Ve is sent to a non-inverting input of amplifier 1. An output of amplifier 1 is connected, via a resistor R in series with a filtering capacitor C, to a supply terminal corresponding to the median potential Vm, for example ground, between supply potentials of amplifier 1. Resistor R has two intermediary terminals for defining two variable resistors for setting, respectively, the cut-off frequency and the gain of the filter. A first intermediary terminal is connected to an inverting input of amplifier 1 and the value of the resistance, between the output of amplifier 1 and its inverting input, sets the cut-off frequency of the filter. A second intermediary terminal of resistor R is used to obtain an output signal Vs, the value of the resistance between the output terminal of amplifier 1 and this second intermediary terminal setting the gain of the filter.
Apart from capacitor C, the value of which (approximately several tens of nanofarads) is too high to enable its integration, all components of the filter generally are integrated within a same circuit, for example an analog mixer. Thus, the operation of a filter such as shown in FIG. 1 requires that the circuit to which it is integrated has a terminal 2 for connecting capacitor C.
A disadvantage of a follower assembly such as shown in FIG. 2 is that it has a high output impedance, since output voltage Vs is obtained via a resistive component. This generally requires the association of the assembly shown in FIG. 2 with an impedance adaptation follower stage.
In a follower assembly such as that shown in FIG. 2, the setting of the value of resistance R between the output terminal of amplifier 1 and the second intermediary terminal of resistor R allows the assembly to operate either as an amplifier or as an attenuator.
FIG. 3 shows a second example of a conventional assembly for an analog audio filter. Here, a differential amplifier 1 mounted as an inverter is used. A non-inverting input of amplifier 1 is connected to a potential Vm generally corresponding to the median potential of a supply voltage (not shown) of amplifier 1. An output of amplifier 1 provides filtered signal Vs and is connected, via a first variable resistor R1, to an inverting input of amplifier 1. The signal Ve to be filtered is sent, via a second variable resistor R2 in series with a capacitor C, to the inverting input of amplifier 1, a fixed resistor R3 being mounted in parallel to the series association of resistor R2 and capacitor C.
In a low frequency mode, that is, for frequencies which are lower than 3 kHz according to the attenuation contour of FIG. 1, the gain of the amplifier is set by the quotient of resistance R1 over resistance R3. Resistance R1 thus is adjusted to correspond to the value of resistance R3 so that the assembly has a unity gain. The high frequency gain of the inverting assembly shown in FIG. 3 corresponds to the quotient of resistance R1 over the parallel association of resistances R2 and R3. Resistance R2 is used to set the cut-off frequency of the filter.
While an inverting assembly such as shown in FIG. 3 overcomes the disadvantages of a follower assembly such as shown in FIG. 2, it has the disadvantage of requiring, in order to be integrated, two terminals, respectively 3 and 4, for connection to capacitor C.
The assembly shown in FIG. 3 corresponds to an amplifier assembly. To implement an attenuator assembly, the series association of resistor R2 and capacitor C is placed in parallel to resistor R1. Upon integration of the assembly, switches are generally used to obtain an amplifying filter or an attenuating filter from a same amplifier 1.
The present invention aims at overcoming the disadvantages of known assemblies, by providing an analog audio filter for correcting high frequencies, the integration of which only requires one connection terminal for a filtering capacitor, while avoiding the use of an additional follower amplifier for adapting the output impedance. The present invention also aims at providing a digitally-controlled active analog filter.
To achieve these objects, embodiments of the present invention provide an analog active filter that includes: a differential amplifier, an output of which issues a filtered signal and a non-inverting input of which is connected to a median potential between supply potentials of the amplifier, a first series association of a first fixed resistor and a first variable resistor between an input terminal of a signal to be filtered and an inverting input terminal of the amplifier, a second series association of a second fixed resistor and a second variable resistor between the output of the amplifier and its inverting input, and a third variable resistor in series with a filtering capacitor, between a midpoint of one of the series associations and the median potential.
According to an embodiment of the present invention, the resistors and the amplifier are implemented in the form of an integrated circuit, a first terminal of the third variable resistor being connected to a terminal for connecting an external capacitor to the integrated circuit, and a second terminal of the third variable resistor being connected to the midpoint of one of the series associations.
According to an embodiment of the present invention, the third variable resistor is connected to the midpoint of the first series association, to form an amplifying filter.
According to an embodiment of the present invention, the third variable resistor is connected to the midpoint of the second series association, to form an attenuating filter.
According to an embodiment of the present invention, the filter includes switching means for selecting the midpoint to which the third variable resistor is connected. According to a further embodiment of the present invention, the switching means are comprised of two CMOS switches.
According to an embodiment of the present invention, each of the variable resistors includes a plurality of fixed resistors mounted in series and associated with switches. According to a further embodiment of the present invention, the switches associated with the series mounted fixed resistors are N-channel MOS transistors. According to a still further embodiment of the present invention, the switches associated with the series mounted fixed resistors are controlled by digital signals, each variable resistor including a same number of switches and each digital signal simultaneously controlling one switch of each variable resistor.
According to an embodiment of the present invention, the filter is applied to correcting high frequencies of an analog audio signal.